CAUTION- MAINS FUSE
TO REDUCE THE RISK OF FIRE REPLACE THE MAINS FUSE ONLY WITH THE SAME
TYPE, WHICH MUST BE A CLASS 3, 230 VOLT, TIME DELAY TYPE, RATED AT 80mA
WHERE THE MAINS INPUT VOLTAGE SWITCH IS SET TO 230 VOLTS AC. AND 160mA
WHERE THE MAINS INPUT VOLTAGE IS 115 VOLTS AC. ALL FUSES MUST COMPLY
WITH BS EN 60127-2:1991, SHEET III. THE FUSE BODY SIZE IS 20mm x 5mm.

CAUTION- MAINS CABLE
DO NOT ATTEMPT TO CHANGE OR TAMPER WITH THE SUPPLIED MAINS CABLE.

CAUTION- SERVICING
DO NOT PERFORM ANY SERVICING. REFER ALL SERVICING TO QUALIFIED SERVICE
PERSONNEL.

WARNING
TO REDUCE THE RISK OF FIRE OR ELECTRIC SHOCK DO NOT EXPOSE THIS
EQUIPMENT TO RAIN OR MOISTURE.

INTRODUCTION
The DS404 is a highly flexible but compact quad gate suitable for a wide
range of professional applications including studio, live sound and
audio installations. The input and output connections are electronically
balanced on XLR connectors and low-distortion VCAs are used to maintain
the highest possible signal quality. Although optimised for +4dBu
operation, the DS404 will work equally well with systems operating at
the -10dBu standard.

Designed to offer the greatest operational flexibility while remaining
very easy to set up and use, the DS404 has two distinct modes of
operation, Hard and Soft, and includes the same powerful side-chain
filter system as pioneered in the Drawmer DS201. Each channel also has
its own external key input.

In the Hard mode, the DS404 functions as a conventional gate with a very
fast, preset attack time (under 10Ás) and a user adjustable release
time. This mode is generally used for processing drums or other
percussive sounds. To prevent the 'chatter' that afflicts some gates, a
hold time is built in which varies depending on the setting of the
Release control while the release curve itself has been modified to
produce a subjectively natural decay response.

In Soft mode, the DS404 functions as a Programme Adaptive expander using
the technology developed for the Drawmer DL241. This 'Programme
Adaptive' expander circuit varies its ratio depending on the dynamics of
the input signal, and in this mode, the attack time continuously adapts
itself to suit the programme material being processed while the release
time remains under control of the user. Because of the progressive
nature of this expander, setting up is far less critical than for most
other gates or expanders making 'Soft' gating ideal for general purpose
cleaning up or for processing material with constantly changing attack
dynamics. In either mode, the attenuation range is switchable to either
-90dB or -20dB.

All four channels may be operated independently or linked for
multi-channel operation. When used independently, any channel may be set
to operate in either mode and with either range setting. When two or
more adjacent channels are linked, the left hand channel becomes the
master and the linked channels respond only to programme material
arriving at the master input.

Each channel has a pair of key-filter controls enabling the
gate/expander action to be made very selective. Essentially, the gate
will respond to frequencies falling between the setting of the Low
filter control and the High filter control enabling the user to choose a
setting that will reject sounds that may otherwise present a risk of
false triggering. To make the setting of these filters almost entirely
instinctive, a key listen facility is provided which allows the filtered
signal to be monitored at the output during setting up. These filters
may also be used as processors in their own right and applications will
be discussed at the end of this manual.

INSTALLATION
The DS404 is designed for standard 19" rack mounting and occupies
1U of rack space. Avoid mounting the unit directly above power
amplifiers or power supplies that radiate significant amounts of heat.
Always connect the mains earth to the unit. Use fibre or plastic washers
to prevent the front panel becoming marked by the mounting bolts.

AUDIO CONNECTIONSBoth the input and output XLRs may be used either balanced or
unbalanced, the wiring convention being: pin 1 ground, pin 2 hot and pin
3 cold. For unbalanced operation, connect pin 3 to ground for both
inputs and outputs, (best achieved inside the actual XLR connector). The
key inputs are unbalanced ║" jack sockets.

If earth loop problems are encountered, disconnecting the mains earth is
not recommended, but instead, try disconnecting some or all of the
signal screens on the cables connecting the DS404 to the patchbay. If
such measures are necessary, balanced operation is advised.

POWER CONNECTIONThe unit will have been supplied with a power cable suitable
for domestic power outlets in your country. For your own safety it is
important that you use this cable. The unit should always be connected
to the mains supply earth using this cable

If for some reason the unit is to be used with a mains input voltage
which is different to that as supplied, this procedure must be carried
out.

Disconnect the unit from the mains.

Using a number 1 size pozidrive
screwdriver, remove the two self-tapping screws holding the voltage
selection switch cover plate on the rear panel.

Remove the cover plate and slide the
switch fully to its opposite end.

Rotate the cover plate one half turn,
(180 ) and refit the two screws.

Replace with a correctly rated fuse
for the selected operation voltage.

CONTROL
DESCRIPTION
All four channels of the DS404 are identical and may be used completely
independently or linked for multi-channel operation. In the linked mode,
only the left most channel's controls are functional and serve as master
controls, though the channel bypass switches remain independent. When
linked, the control signal is derived only from the programme material
present at the input of the master channel.

Threshold Sets the level below which
gating starts to take place and may be set in the range -70dB to +20dB.

Display When the gate is closed, the
red LED above the Threshold control is illuminated; when the gate is
open the green and amber LEDs come on and the red one goes off. When the
input signal falls below the threshold, the green LED will extinguish
and the amber LED will fade over the duration of the release time. This
is the same 'traffic light' system used on the Drawmer DS201.

Note: In soft mode, because the expansion ratio varies with signal
level, it is possible to arrive at a situation where the red LED may be
illuminated when little or no perceived gain reduction is taking place.
This means that the expansion Threshold may need to be set a little
higher than in the Hard mode and final setting is best done by ear. The
adaptive expansion system used in the DS404 means, however, that setting
up is not over-critical in Soft mode.

Release The release time may be set
from 10mS to 5S and the control operates in a similar manner whether in
Soft or Hard mode. A hold time is built into the system which varies
with the release time setting. This is to prevent chatter when
processing material with inconsistent decay characteristics and is quite
invisible to the user. As a general rule, the release time should be set
as fast as possible while ensuring that slowly decaying sounds, or
sounds which include a lot of reverberation are not significantly
shortened.

Range Determines how much gain
reduction is applied when the gate is fully closed. The -90dB setting
effectively silences the channel completely when the gate is closed
while the -20dB setting will still allow an attenuated version of the
channel signal to pass through.

Key Listen / Gate / Bypass When this
switch is set to Key listen, the effect of the key filters on the
programme material is heard at the output. In normal operation, the Gate
position is selected; the filters only affect the way the DS404 responds
to the incoming programme material - they do not have any direct effect
on the output signal. However, these filters can be useful in certain
applications in which case the switch may be left in the Key Listen
position during operation. This will cause the audio signal to be routed
through the filters but not through the gate. The Bypass position routes
the input signal to the output with no processing.

L.F. The L.F. or Low Frequency
filter is variable from 25Hz to 10kHz and works by severely attenuating
frequencies below the cut-off frequency selected.

H.F. Variable from 200Hz to 35kHz,
this filter attenuates frequencies above the selected cut-off value. In
other words, when both filters are set, it is the range between the two
settings that is allowed to pass.

Note: Any side-chain filtering which implements high frequency
attenuation will also cause a slight delay in the time the gate takes to
trigger. Under most circumstances this will be quite imperceptible, but
when transient sounds are being processed with the H.F. control set to a
very low value, some degradation of the attack transient may become
apparent. For this reason, always set the H.F. control to the highest
possible value practicable when processing percussive sounds.

Key/Int In the Int position, this
switch causes the gate to respond to the dynamics of the signal present
at the main signal input socket. In the Key position, an external audio
source may be used to control the gate action making it possible to gate
one sound according to the signal dynamics of another, independent
signal.

Hard/Soft This button selects
between the Hard and Soft modes of operation. In the Hard mode, the
DS404 behaves as a fast, conventional gate while in Soft mode, it
performs as a programme adaptive expander. Hard mode is signified by a
red status LED while the Soft mode is identified by a yellow LED. If two
or more channels are linked, the mode is determined by the master (left
most) channel and the Hard/Soft LEDs on the slave channel(s) will
extinguish.

Slave Link The Link buttons are
located between channels, and when depressed, cause the channel on the
right to be controlled by the left hand channel. Two, three or four
channels may be linked, (or two pairs), the left most channel always
being the master. In linked mode, the red status LED beneath the Slave
Link switch will be illuminated, and both the yellow and red HARD/SOFT
LEDs of the slave channel will be extinguished. This assists to show
that the only controls of the slave channel that still function are the
Key Listen / Gate / Bypass switch and the Range switch.

OPERATIONThe unit should be connected in-line with the signal to be
processed via suitable insert points. Ensure, where possible, that the
insert send and return level on your console approximately matches the
operating level of the DS404.

For mono use, each channel may be considered as being completely
independent and set up accordingly. For use with stereo or multi-channel
signals, two or more channels may be linked; all setting up is then done
using the left hand channel's controls in any linked group.

For percussive material such as drums, fast synthesised sounds or
percussive bass guitar, the Hard mode of operation will give the fastest
response. However, with material having a slower attack, the Hard
setting may produce transient clicks at the beginning of sounds in which
case the Soft setting may produce better results.

Initially, the Low filter should be set fully anti-clockwise while the
High filter should be set fully clockwise. This will allow the full
audio spectrum of the input programme to be monitored by the side-chain
control circuitry. Set the Range control to -90dB and the output
selector switch to Normal.

With the release control set at its mid-way position, Range set to -90dB
and with suitable programme material fed into the DS404, increase the
Threshold level from its anti-clockwise position until the gate starts
to operate. This will be indicated by the activity of the traffic light
LEDs and you should also hear the effect on the outputs signal in that
pauses in the programme will now be silent. If the threshold setting is
too high, the gate will start to cut out wanted pieces of programme so
you should adjust it to as low a setting as possible consistent with the
effective removal of low level noise. If the ends of sounds are
obviously being truncated, then a longer release time may help. On the
other hand, if unwanted noise is audible after the wanted sound has
ended, a shorter release time may well be more appropriate.

There are circumstances when the programme material is corrupted not
only by unwanted random noise, but by some other sound. For example, in
a multi-miked drum kit setup, some hi-hat will inevitably leak into the
snare mic, some snare drum into the kick drum mic and so on. Equally,
when recording on location, you may experience problems due to wind or
traffic noise or close-by conversation. If the unwanted noise is
different in pitch to the wanted sound, it is often possible, by using
the Key Listen facility, to use the filters to 'tune' into the wanted
sound while excluding as much of the unwanted sound as possible. Used
carefully, these filters can significantly increase the gate's immunity
to false triggering.

LINKED OPERATIONIt is important to note that when two or more channels are
linked, the control signal is derived entirely from the left-hand
channel in the group and not from a mix of the individual channels. This
means that stereo signals where one channel differs significantly from
another may fare better if the channels are not linked.

On the other hand, this mode of linking is very powerful in
synchronising the start and finish of sounds, a typical application
being to tighten up backing vocals. If one singer tends to finish notes
on time while the others hang on too long, the correct version can be
used as the master to ensure that all the others finish at the same
time. Similarly, one sound can be used to gate another without having to
resort to patching in an external key signal. An example might be to
gate a low frequency tone from a bass drum signal and then add this
gated tone to the drum sound to add depth.

DIFFICULT MATERIALAs with any other gate, noise can only be removed during
pauses in the wanted material. If the noise contamination is serious
enough to be evident even during moderately loud programme material,
then simple gating will do little to help. Indeed, the very fact that
the gate produces near-perfect silence during pauses can make the noise
content of the programme material seem even worse. In marginal cases,
setting the range control to -20dB rather than -90dB will adequately
reduce the noise during pauses but not sufficiently to cause an
unacceptably dramatic change in noise level as the gate opens and
closes.

More sophisticated processors such as the Drawmer DF320 are better able
to cope with excessive noise as they adaptively filter the programme so
as to mask the noise during low level passages or where there is little
high frequency content present to mask it. However, the key filters in
the DS404 may also be used to good effect, particularly in situations
where the wanted signal does not occupy the full audio spectrum.

Taking the example of the electric guitar, this produces little below
100Hz or above 3kHz so setting one channel of the DS404 to Key Listen
mode will enable you to use the filters to exclude much of the amplifier
hum at the low end and hiss at the top end while having little effect on
the sound of the guitar. Surprisingly, the same is true of the acoustic
guitar; (even a bright-sounding steel-strung model), and the filters can
be used to reduce the effect of string squeak or the player's breathing.

Other applications of the filter section include shaping DI'd electric
guitar sounds to remove unpleasant overtones and to simultaneously clean
and warm up digital synthesizer sounds. While we should always endeavour
to get the highest quality of programme material at source, every
engineer is occasionally confronted with inferior material from sources
beyond his control. Conventional equalisers seldom have a sharp enough
response do duplicate the function of the Key filters which invariably
results in the wanted material being filtered as well as the unwanted
noise.

Once a signal has been filtered in this way, it may also be processed by
another channel of the DS404 to apply conventional gating.